I put my library books on the table. Atop the stack was a book titled, Polio: An American Story. My youngest son, then 15, glanced at it. Puzzled, he asked, “Dad, what’s polio?”

For me, born in 1955, the year Jonas Salk developed his vaccine, polio was an ugly scar ripping through my childhood. Forty years later, my son did not know what it was. To him, polio, like measles, diphtheria, whooping cough, and tetanus, were causes for inconvenience at his annual checkup.

Yet these diseases are still out there, as contagious—and deadly—today as in my youth. They are held at bay through vaccination. Virtually every contagious disease can be controlled through vaccination.

Vaccination is not risk-free. The risks of any vaccine that controls diseases spread by airborne, waterborne, or food-borne contact is trivial, though when compared to the effects of an epidemic. Not everyone believes that—until an epidemic strikes. Polio swept northern Nigeria in 2003. Local imams told parents that polio vaccines caused HIV or were a western plot to sterilize African children. Afterwards, many parents rejected the vaccines. Someone from Nigeria caught polio traveling abroad. Brought home, it spread quickly among the unprotected population. Hundreds, mostly children, caught polio.

This could not happen here in the United States, you say, because we are one of the worlds’ most advanced nations? Disease does not respect national boundaries, technology levels, or parental fears about vaccination. Epidemics can happen wherever large unimmunized populations exist, including the United States.

Diseases require a minimum number of hosts. When that pool gets too small, the disease disappears. We have been immunizing for so long that many diseases are gone domestically. If no one brings the disease into here, no one catches it.

Herd Immunity

If you do not immunize your children, they do not run the risks posed by vaccination. If everyone else immunizes their children, your children are safe—the pool of hosts is too small to matter. Your children are protected by “herd immunity.” Until enough others parents figure this out, too. They too, let the other parents take the risks associated with immunization and stop immunizing their children. When enough people do that, things get explosively dangerous.

In January 2008, a boy from California traveled to Switzerland. He had not been vaccinated for measles. Perhaps his parents thought getting vaccinated was too risky. There was a measles outbreak in Switzerland. When the boy returned home, he had measles. A dozen children exposed to this child quickly caught measles. Four caught the disease sharing a doctor’s waiting room with the sick child. None were immunized. Of 376 children at the infected child’s school, 36 lacked immunization. Five of these—over 13%—caught measles. None of the 340 immunized children got measles.

Measles rarely kills, but there can be complications. Other contagious diseases are less benign. Do not ignore a large risk in an effort to avoid a lesser one. Immunize your children. Keep “What’s polio?” a question that children ask.